This application relates to the inspection of multiply layered light transmissive structures, and particularly to a novel method for detecting defects beneath the surfaces of such structures.
The usual processes for the inspection of products such as photographic film and the like for defects consist primarily of inspection for surface defects, as by the use of a flying spot scanner and a reflection or transmission detector. U.S. Pat. No. 3,026,415 shows apparatus for detecting defects on webs such as photographic film comprising a flying spot scanner, and describes circuits for processing the signals from the scanner to eliminate noise and characterize particular defects. In this patent, the usual preference for the use of light of a wave length that will not affect the film being inspected is indicated. Other patents of interest in connection with the flying spot scanning of webs for defects are U.S. Pat. Nos. 3,589,817; 3,646,353 ; and 3,748,047.
In U.S. Pat. No. 3,556,664, it is stated that, particularly for colored film and like products, the use of infrared light, to which the emulsions will not respond, is not adequate to reveal subsurface defects by transmission methods. Instead it is proposed to use actinic light at a sufficiently low exposure level that the reciprocity failure characteristics of the emulsions will prevent damage during inspection.
Another approach to the use of visible light for defect detection is described in U.S. Pat. No. 3,734,624. A flying spot scanner is provided in which the flying beam is polarized so that essentially all of the beam will be reflected from the web being inspected upon illumination at a grazing angle of incidence. In this way, visible light can be used for inspection without any penetration of light into the emulsion that would expose the emulsion.
U.S. Pat. No. 3,994,586 discusses a method of measuring the thickness and uniformity of a translucent film by the detection of differences in intensity between a first beam of light at an infrared frequency which is not absorbed by the film material, and a second beam of light that is selectively absorbed by the material. This patent is directed to a problem encountered in conventional reflectance measurements, which is caused by interference between a portion of the illuminating beam falling on the surface and reflected therefrom, and a residual portion of the beam that passes through the material and is reflected from the next surface below, providing an interfering difference in path length. The patent proposes polarizing techniques for removing the unwanted component from a reflected beam to prevent such reflectance interference signals.
So far as is known, prior to the present invention, the only method proposed for detecting subsurface defects in photographic film is that disclosed in the above-cited U.S. Pat. No. 3,556,664, using light transmitted through the emulsions. This method requires very low exposure levels, and correspondingly reduced sensitivity. Moreover, it is inapplicable to opaque film structures, such as those incorporating subtractive dye systems, or those having an opaque base. The object of this invention is to facilitate the detection of subsurface defects in structures such as color films and the like, which can be employed even with structures that will not permit the passage of light throughout the structure, but only through one or more layers.
Briefly, the above and other objects of the invention are attained by a process in which a multilayered web is illuminated by a beam of light that is partially reflected and partially transmitted through each of the several layers of the structure. The reflected beam is focused on a detector, producing a combined reflectance interference signal. This signal is a function of the thicknesses and indices of refraction of the several layers and of the light scattering and light absorbing properties thereof. It has been found that signals produced in this manner will indicate even very small subsurface defects, such as bubbles, streaks and the like, which may occur. Infrared light, to which emulsions are insensitive, may be used. Thus, a much more intense beam can be used than could be tolerated with methods employing visible light. Preferably, the web is moved in one direction while the beam is moved across it in a perpendicular direction by a flying spot scanner.
The invention will best be understood in the light of the following detailed description, together with the accompanying drawings.